The Mexican-American War: Causes, Conflicts, and Consequences
The Mexican-American War (1846-1848), also known as the Mexican War, stemmed from the US annexation of Texas in 1845 and a border dispute between the Nueces River (Mexico’s claim) and the Rio Grande (US claim). The war resulted in the US acquiring over 500,000 square miles of Mexican territory, extending from the Rio Grande to the Pacific Ocean.
Key Questions about the War
What were the primary causes of the Mexican-American War?
How did Manifest Destiny influence the war?
Was there opposition to the war in the United States?
What territories did the US gain from the war?
How did the war affect sectionalism in the US?
Prelude to War: Polk and the Texas Border Dispute
Following the US annexation of Texas, Mexico broke off diplomatic relations in March 1845. In September, President James K. Polk sent John Slidell to Mexico City to negotiate the border dispute, settle US claims against Mexico, and potentially purchase New Mexico and California. Mexican President José Joaquín Herrera refused to receive Slidell, leading Polk to order General Zachary Taylor to occupy the disputed land between the Nueces and Rio Grande in January 1846.
Proclamation by President James K. Polk declaring war against Mexico (1846). 
Portrait of James K. Polk.On May 9, 1846, Polk prepared a message to Congress regarding the potential for war. News arrived that Mexican troops had attacked Taylor’s forces on April 25. Polk’s revised message to Congress on May 11 stated that Mexico had “invaded our territory and shed American blood on American soil.”
Opposition to the War: Spot Resolutions and Civil Disobedience
Despite Congressional approval for war on May 13, the US was divided. Democrats, particularly in the Southwest, supported the war, while many Whigs viewed it as an unjust land grab. Whigs questioned the location of the initial conflict and Polk’s right to declare war. The debate centered on the true border between the two nations and the validity of Polk’s claims.
Soldier’s Adieu, lithograph depicting public enthusiasm for the war (1847).
Photograph of Abraham Lincoln, 1846.Future president Abraham Lincoln, then a Congressman, challenged Polk’s justification for war. In December 1847, Lincoln introduced the “Spot Resolutions,” demanding proof of the attack’s location and questioning the validity of Polk’s claims.
Get exclusive access to content from our 1768 First Edition with your subscription. Subscribe today
Lincoln’s resolutions aimed to expose inconsistencies in Polk’s narrative and highlight the questionable nature of the war’s origins.